"Nanoions": fundamental properties and analytical applications of charged nanoparticles.

Mixtures of oppositely charged nanoparticles (NPs) precipitate sharply only at the point of NP electroneutrality. This behavior-reminiscent of the threshold precipitation of inorganic ions-is specific to the nanoscale and can be attributed to the formation of like-charged NP clusters stabilized in solution by mutual electrostatic repulsions. NP titrations based on this phenomenon provide a uniquely accurate tool for measuring charges tethered onto nanoscopic objects and for studying the thermodynamics of surface reactions at the nanoscale.